Re: Compression Software

[Ken K <ellipsis@concentric.net>]

BlkSwan03@aol.com wrote:
> 
> Does anyone on the list use compression software (like Stuffit Deluxe) to
> squeeze as much as you can onto your drives?  Does this create problems or
> mess up quality?

Stuffit - a lossless compression method - does not alter your data. Many
people use it (or PKZIP on the PC) when they archive seldom-used files.

When you are compressing images with an image editing program, JPEG
compression will definitely alter data (lossy compression). If you have
an image in JPEG format, and you resave it as JPEG, the data alteration
compounds. JPEG is a good compression algorithm, and it tries to only
throw away data that is least perceptible to the human eye, but it does
permanently change the image each time it's used. Therefore, use it
once, and only once. You will see many images on CD collections that are
supplied in JPEG format. This is fine, since the images are usually very
high resolution, and are intended for printing at resolutions of 200 ppi
or higher. At these settings, the effects of JPEG are barely noticeable,
if at all. If you make any alterations to such an image, you should
resave it in a format which uses no compression, or at least one that
uses a lossless method. LZW compression is an example of lossless
compression.

Where JPEG will catch up tp you is when it is applied to an image which
will be viewed on-screen, at screen resolution. In that case the
different compression ratios have noticeably different effects on the
viewing quality of the image.

Ken Kehl
ellipsis design
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